Wind turbines operate at either a constant rotational speed or at variable rotational speeds that are proportional to the wind velocity. Peak power at high wind speeds is usually controlled through stall regulation or through the use of variable pitch turbine blades. A conventional horizontal axis wind turbine (HAWT) employed to generate electric power typically includes two or more turbine blades each associated with a central hub. The hub rotates about an axis and is connected to a shaft. Conversion of wind power into electrical power is accomplished in most wind powered systems by connecting the shaft to drive an electric generator.
The point of the turbine blade closest to the hub is called the root of the blade, while the point of the turbine blade farthest from the hub is called the tip of the blade. The portion therebetween is the mid-span. A line connecting root to tip is referred to as the span of the blade. A cross-section of a turbine blade taken perpendicular to the span is generally referred to as an airfoil. Theoretically, therefore, each turbine blade includes an infinite number of airfoils along that line and it is the collection of airfoils that fully describes the blade's contours and shape. Typically, however, a blade's shape is defined in reference to a finite number of the airfoil shapes for convenience. Further, once the airfoils are determined, it is the accepted practice that at least some portion of the blade is further designed by application of a computer program that interpolates between the fixed airfoils to create foils therebetween.
Blade design starts with airfoil shapes. Next, computer programs have been employed to complete the design of the blades. However, employing these programs has created problems with wind turbines. Allowing a CAD program to loft the blade surface for airfoil sections which have been arbitrarily placed for optimal aerodynamics may result in waves or ripples in the surface loft that can be deleterious to structural integrity. This occurs when the CAD Program forces a surface through the pre-defined airfoil section. In addition, the thickness of blades designed using CAD programs are often kept artificially low in order to minimize the computer program's negative effects on the curvatures between fixed airfoil stations. However, keeping the thickness low often results in blades that may not be able to bear the loads required; these blades may buckle. On the other hand, incorrectly assigned airfoil coordinates for thicker blades can result in less than desirable aerodynamic properties in some portions of the blade. Therefore, many blades used in wind turbines often sacrifice structural soundness and dependability in exchange for more aerodynamic attributes. Since the primary goal of a wind turbine is to convert the kinetic energy of the wind into electrical energy as inexpensively and efficiently as possible, operational efficiency of the wind turbine is negatively affected by a structure that can allow the blade to buckle under certain loads. The blade design of the present invention addresses these problems without the corresponding expected loss in aerodynamic character.